Petroleum distillate streams contain a variety of organic chemical components.
Generally the streams are defined by their boiling ranges, which determine the composition. The processing of the streams also affects the composition. For instance, products from either catalytic cracking or thermal cracking processes contain high concentrations of olefinic materials as well as saturated (alkanes) materials and polyunsaturated materials (diolefins). Additionally, these components may be any of the various isomers of the compounds.
The composition of untreated naphtha as it comes from the crude still, or straight run naphtha, is primarily influenced by the crude source. Naphthas from paraffinic crude sources have more saturated straight chain or cyclic compounds. As a general rule most of the “sweet” (low sulfur) crudes and naphthas are paraffinic. The naphthenic crudes contain more unsaturates, cyclic, and polycylic compounds. The higher sulfur content crudes tend to be naphthenic. Treatment of the different straight run naphthas may be slightly different depending, upon their composition due to crude source.
Reformed naphtha or reformate generally requires no further treatment except perhaps distillation or solvent extraction for valuable aromatic product removal. Reformed naphthas have essentially no sulfur contaminants due to the severity of their pretreatment for the process and the process itself.
Cracked naphtha, as it comes from the catalytic cracker, has a relatively high octane number as a result of the olefinic and aromatic compounds contained therein. In some cases, this fraction may contribute as much as half of the gasoline in the refinery pool together with a significant portion of the octane.
Catalytically cracked naphtha gasoline boiling range material currently forms a significant part of the gasoline product pool in the United States and is the cause of the majority of the sulfur found in gasoline. These sulfur impurities may require removal in order to comply with product specifications or to ensure compliance with environmental regulations, which may be as low as 10, 20 or 50 wppm, depending upon the jurisdiction. For example, in the United States, Tier II gasoline regulations currently require refiners to achieve 50-60 ppm S in the FCC gasoline, which necessitates a conversion of approximately 90%-97% S. The EPA is now considering Tier III ultra-low sulfur gasoline regulations, requiring less than 10 ppm S. This will increase conversion requirements up to 98%-99.5%.
The most common method of removal of the sulfur compounds is by hydrodesulfurization (HDS) in which the petroleum distillate is passed over a solid particulate catalyst comprising a hydrogenation metal supported on an alumina base. Additionally, large amounts of hydrogen are included in the feed. The hydrodesulfurization reaction results in the production of hydrogen sulfide according to the following reaction: RSH+H2R′+H2S. Typical operating conditions for standard single pass fixed bed HDS reactors, such as in a trickle bed reactor, are temperatures ranging from 600° F. to 780° F., pressures ranging from 600 to 3000 psig, hydrogen recycle rates ranging from 500 to 3000 scf/bbl, and fresh hydrogen makeup ranging from 100 to 1000 scf/bbl.
After the hydrotreating is complete, the product may be fractionated or simply flashed to release the hydrogen sulfide and collect the desulfurized naphtha. In addition to supplying high octane blending components the cracked naphthas are often used as sources of olefins in other processes such as etherifications, oligomerizations, and alkylations. The conditions used to hydrotreat the naphtha fraction to remove sulfur will also saturate some of the olefinic compounds in the fraction, reducing the octane and causing a loss of source olefins. Unfortunately, the operating severity required by existing units to achieve 10 ppm S will incur much higher octane losses. The loss of olefins by incidental hydrogenation at the severe conditions is detrimental, reducing the octane rating of the naphtha and reducing the pool of olefins for other uses.
Various proposals have been made for removing sulfur while retaining the more desirable olefins. Because the olefins in the cracked naphtha are mainly in the low boiling fraction of these naphthas and the sulfur containing impurities tend to be concentrated in the high boiling fraction, the most common solution has been prefractionation prior to intensive hydrotreating. The prefractionation produces a light boiling range naphtha which boils in the range of C5 to about 150° F. and a heavy boiling range naphtha which boils in the range of from about 250-475° F.
Two prior art methods that have been used to reduce the sulfur content of gasoline to 10 ppm are illustrated in FIGS. 1 and 2. The simplified process flow diagrams of FIGS. 1 and 2 illustrate major components of the process, and additional components may be present, such as pumps, heat exchangers, condensers, reboilers, hot drums, cold drums, etc., as would be understood by one skilled in the art.
Several U.S. patents describe the concurrent distillation and desulfurization of naphtha, including U.S. Pat. Nos. 5,597,476; 5,779,883; 6,083,378; 6,303,020; 6,416,658; 6,444,118; 6,495,030 and 6,676,830. In each of these patents, the naphtha is split into two or three fractions based upon boiling point or boiling ranges. Two methods that have been used to reduce the sulfur content of gasoline to 10 ppm are illustrated in FIGS. 1 and 2. The simplified process flow diagrams of FIGS. 1 and 2 illustrate major components of the process, and additional components may be present, such as pumps, heat exchangers, condensers, reboilers, hot drums, cold drums, etc., as would be understood by one skilled in the art.
One such process is illustrated in FIG. 1. A full range cracked naphtha 10 is fed to a first catalytic distillation column 12 having a bed 14 containing a thioetherification catalyst in an upper portion of the column. The full range naphtha is fractionated to form a heavy fraction 16 (including medium cracked naphtha and heavy cracked naphtha) and a light fraction 18 (light cracked naphtha), and the dienes and mercaptans are reacted in bed 14 to produce thioethers, which are recovered with the heavy fraction. The heavy fraction 16 is then fed to a second catalytic distillation column 20, having beds 22, 24 containing hydrodesulfurization catalysts, where the medium and heavy cracked naphtha fractions are separated and hydrodesulfurized. The heavy and medium cracked naphthas, following desulfurization, are recovered as overheads and bottoms fractions 26, 28, respectively, fed to separator 30 to remove dissolved hydrogen sulfide 32, and then fed via flow line 34 to a fixed bed reactor 36 containing a bed 38 of hydrodesulfurization catalyst to react the heavy and medium naphtha fractions and further reduce the sulfur content of the combined fractions. To significantly reduce the sulfur content of the heavy and medium cracked naphtha fractions to meet a 10 ppm S specification, harsh conditions are generally required in reactor 36, which may result in significant losses of olefins and an undesirable loss in octane rating for the combined product 40.
As illustrated in FIG. 2, a full range cracked naphtha 50 and hydrogen 52 may be fed to a selective hydrogenation unit 54 to hydrogenate dienes and react mercaptans with dienes to form thioethers. The effluent 56 is then fed to a separator 58 to separate the full range cracked naphtha into a light cracked naphtha fraction 60 and a heavy fraction 62 (including medium and heavy cracked naphtha). Hydrogen 63 and heavy fraction 62 is then fed to a fixed bed reactor 64 containing a bed 66 of hydrodesulfurization catalyst to react the heavy and medium naphtha fractions and further reduce the sulfur content of the combined fractions. To significantly reduce the sulfur content of the heavy and medium cracked naphtha fractions to meet a 10 ppm S specification, harsh conditions are generally required in reactor 64, which may result in significant losses of olefins and an undesirable loss in octane rating for the combined product 68.